Induction heating systems



Sept. 27, 1960 J. REBOUX mnucrrou HEATING SYSTEMS Filed Feb. 5, 1959 IERN REB UX 2,954,453 Patented Sept. 27, 1960 INDUCTION HEATING SYSTEMS Jean Reboux, Massy, France, assignor to Societe de Traitements Electrolytiqnes & Electrothermlques, a corporation of France Filed Feb. '3, 1959, Ser. No. 790,841

Claims priority, application France Feb. 7, 1958 Claims. (Cl. 219-10.75)

The present invention relates to induction heating systems. More particularly, it relates to systems for heating glass fibre extruding spinners.

It is known to manufacture glass fibre by extruding molten glass through minute holes provided in rotating dish shaped spinners. Such spinners are usually heated by means of gas burners to a temperature of say 1,000 C.- As is well known to those skilled in this particular art, it is essential that this temperature should remain constant on the whole of the spinner surface. However, this cannot be achieved by gas heating and a difference of at least 100 C. appears between the top and the bottom of the spinner, if only gas burners are used. For many practical reasons, such a temperature difference is not commercially acceptable and auxiliary heating sources must be used to compensate therefore and to keep the bottom of the spinner at the same temperature as the top thereof.

This has been done so far by inducing in the spinner low frequency currents. The applicant has discovered that by using high frequency currents much better results are provided and a thorough consideration of this delicate problem has shown that, with spinners made of 2 millimeters thick special steel, the suitable frequency was comprised between 300 and 500 kilocycles per second, and was generally of about 300 kilocycles.

However, the provision of an ultra high frequency generator, capable of being used for heating a fibre glass spinner, creates difficult problems. For many reasons it is utterly unpracticable to use a separate high frequency generator and to apply the energy it provides, say through a coaxial cable to an induction coil. On the other hand it is obvious that because of the high temperature to which the spinner and the induction coil are submitted, it is necessary to keep the oscillator at a substantial distance from the oven where the spinner is located during its operation.

It is also of paramount importance to have a heating source which needs no adjustments during the operation of the spinner. The latter operates in a continuous manner and it would not be practical to adjust the oscillation frequency, should the impedance of the spinner vary in the course of its operation. In this respect, it should be noted that spinners are extremely expensive devices. A variation of temperature, due for example to an inadequate matching of the spinner to the generator, readily results in such a temperature ofthe spinner that the latter breaks. p

Therefore it is an object of the invention to provide a high frequency heating system, more particularly adapted for providing a supplementary heating for glass fibre extruding spinners. In brief a system according to the invention comprises a high frequency generator having an oscillatory circuit whose coil comprises at least two branches in parallel, one of which is used as an induction coil surrounding the base of the spinner,

the capacitor of the oscillator circuit being connected in parallel with said branch.

The invention will be better understood from the following description taken in conjunction with the appended drawing, which is given solely by way of example and wherein,

Fig. 1 is a diagrammatic view of a high frequency heating equipment according to the invention;

Fig. 2 is an embodiment of the connection between the coaxial cable and the heating coil; and

Fig. 3 shows an embodiment of the heating inductor.

Referring to Fig. l a cabinet comprises a triode 2 including a cathode 3, a grid 4 and a plate 5, the cathode being earthed. The grid and the anode circuits are coupled to each other by the mutual inductance 12 of coils 13 and 14 which are respectively connected in the grid and in the plate circuit of triode 2. One terminal of coil 13 is earthed, its other terminal being connected to grid 4 through a circuit including a capacitor 8, which is placed in shunt across a resistance 9. A capacitor 7 is connected between the cathode and the grid. One terminal of coil 14 is earthed, its other terminal being connected to a circuit comprising in parallel a variable capacitor 10 and an adjustable inductance coil 11. This circuit is connected to anode 5 through a capacitor 6.

The inductor assembly 17 comprises an inductor 18, the ends 31 and 30 of which are connected to points 21 and 22 respectively. The oscillator triode 2 has its plate circuit connected to inductor 18 through a co-axial cable 1516, the envelope of which is grounded.

According to the invention, coil 18 comprises a slidable contact 23 which is connected to a terminal 20, which in turn, is connected to the central conductor 24 of the coaxial cable 16. Point 22 is grounded and point 21 is grounded through a capacitor 19.

Fig. 2 illustrates the method of connecting the movable assembly 17 and the coaxial supply line 16. The ends of the three conductors, respectively connected to ends 30 and 31 of coil 18 and to contact 23, are in the shape of rigid rods 201, 211 and 221 respectively adapted to be inserted into rigid copper jaws 202, 212 and 222,

fixed onto terminals 20, 21, 22. To insure a better contact the jaws are provided with terminal studs. Only two of such studs, namely studs 203 and 204, 213 and 214, 223 and 224 are shown in each jaw. They are made of a special and highly elastic alloy. Even a reduced number of these contact studs, for example three to five, with contact surfaces of only a few square millimeters, insures a good current up to about 200 to 400 amperes. This contact system is much more simple and reliable than the conventional pneumatic, mechanic or electromagnetic clamping arrangements so far used. It will be appreciated that coil 18 may thus be readily disconnected from the coaxial cable 16, for example in case of breakdown or failure or if it is desired to employ another heating inductor or for any of other reason.

It will be appreciated that coils 18 and 11 are in parallel. The inductance of coil 18 is substantially lower than the inductance of coil 11, say within a ratio of one to ten and therefore the operating frequency of the system is in fact determined by the circuit comprising coil 18 and capacitor 19.

The reaction circuit of the oscillator triode 2 is practically independent of the circuit comprising coil 18 and capacitor 19 and of the operating frequency of this circuit. Magnetically coupled coils 1314 provide the coupling between grid and plate circuits. Of course the reaction circuit voltage increases with the operating fre quency. To avoid any outside adjustment of the reaction voltage, the suitably preset capacitor 10 is used. It

is adjusted to increase impedance of the circuit comprising coil 11 and capacitor in parallel, thus reducing the current flowing through coil 14 and accordingly the reaction voltage; For an inductance value of coil 11 of the order of 50 p.11. experience shows that capacitor 10 shouldhave. a capacity comprised between 60 and 100 pf, thus providing an excellent voltage stability, whatever the operating frequency in the 2001500 kc./s. range.

It is generally advantageous that the output voltage of oscillator 1 he of a rather moderate value, i.e. should not exceed 5000 to 6000 volts. A much higher voltage, for example up to 15,000 volts, may be needed across the inductor coil18 and that is why this coil is advantageously mounted as an auto-transformer as shown in Fig-.1.

Fig. 3 shows how inductance coil may be associated with a spinner 2.5. The latter is shaped as an inverted disk having minute holes distributed over the whole of its surface. The molten glass, which is fed to the spinner through the upper portion thereof, is extruded through the holes as the spinner rotates. The spinner is brought to a temperature of about 900 to 1200 C. and is rotated about its axis at a great velocity. As already mentioned, the heating is generally carried out by means of gas burners. Experience shows that the temperature along any generatrix of the plate is in this case generally not homogeneous. The existence of a temperature gradient is very embarrassing.

In order to obviate this drawback, the conventional heating system is supplemented by high frequency heating, as explained above. To this end, inductor coil 1% is positioned under the spinner with its upper surface level with the bottom of the spinner, as shown in Fig. 3. The coil of the inductor is formed by a tube 27 which is cooled by water circulating therein. Tube 27 is wound about a quartz cylinder 26, the outer surface of which is provided with grooves wherein tube 27 is rigidly maintained. Quartz has a good heat resistance and is excellent dielectric. The structure thus provided is particularly sturdy and affords good protection against any contact of theinductor with fibre jets, which would otherwise rapidly wear out the inductor.

It is to be understood that the invention is in no way limited to the embodiment illustrated which is given only by way of example.

What is claimed is:

1. A high frequency induction heating system comprising: a high frequency oscillator including an oscillator tube having an anode circuit: an oscillatory circuit in said anode circuit comprising a capacitor, a first induc tion coil and a second induction coil, means coupling the second induction coil in parallel with said capacitor, and means coupling said induction coils in parallel, the second induction coil having a substantially lower induction than said first induction coil; and a transmission line for connecting said second coil to said oscillator tube, said first coil being directly connected to said oscillator.

2. A high frequency induction heating system comprising: a high frequency oscillator including an oscillator tube having an anode circuit and a grid circuit having respective coils for coupling said circuits to each other; an oscillatory circuit in said anode circuit comprising a capacitor, a first induction coil and a second induction coil, means coupling the second induction coil in parallel with said capacitor, and means coupling said induction coils in parallel, the second induction coil having a substantially lower induction than said first induction coil; said tube, grid circuit and first induction coil forming a first assembly and said second coil and capacitor forming a second assembly; and a transmission line for connecting said assemblies to each other.

3. A high frequency induction heating system comprising: a high frequency oscillator including an oscillator tube having an anode circuit and a grid circuit having respective coils for coupling said circuits to each other: an oscillatory circuit in said anode circuit comprising a capacitor, a first induction coil and a second induction coil, means coupling the second induction coil in parallel with said capacitor, and means couplingsaid induction coils in parallel, the second induction coil having a substantially lower induction than said first induction coil; said tube, grid circuit and a first induction coil forming a first assembly and said second coil and capacitor forming a second assembly; a transmission line for connecting said assemblies to each other; and an additional capacitor in parallel with said first inductance coil for reducing the voltage induced by said plate circuit in said grid circuit upon the frequency of the oscillator increasing.

4. A high frequency induction heating system comprising: a high frequency oscillator including an oscillator tube having an anode circuit and a grid circuit having respective coils for coupling said circuits to each other; an oscillatory circuit in said anode circuit comprising a capacitor, a first induction coil and a second induction coil, means coupling the second induction coil in parallel with said capacitor, and means coupling said induction coils in parallel, the second induction coil having a substantially lower induction than said first induction coil; said means for coupling the second induction coil to said capacitor, an inductor being incorporated into said inductor; said tube, grid circuit and first induction coil forming a first assembly and said second coil and capacitor forming a second assembly; a transmission line for connecting said assemblies to each other; and an additional capacitor in parallel with said first inductance coil for reducing the voltage induced by said plate circuit in said grid circuit upon the frequency of the oscillator increasing.

5. A high frequency induction heating system comprising: a high frequency oscillator including an oscillator tube having an anode circuit and a grid circuit having respective coils for coupling said circuits to each other; an oscillatory circuit in said anode circuit comprising a capacitor, a first induction coil and a second induction coil, means coupling the second induction coil in parallel with said capacitor, and means coupling said induction coils in parallel, the second induction coil having a substantially lower induction than said first induction coil; said tube, grid circuit and first induction coil forming a first assembly and said second coil and capacitor forming a second assembly; a transmission line for connecting said assemblies to each other; an additional capacitor in parallel with said first inductance coil for reducing the voltage induced by said plate circuit in said grid circuit upon increase of the oscillator frequency; an inductor comprising a heat resistant armature and a tubular winding supported by said armature, said armature being adapted for being placed under a rotating glass fibre extruding spinner, level with the bottom thereof, for providing a supplementary source for heating said spinner, said inductor comprising said second coil.

References Cited in the file of this patent UNITED STATES PATENTS 

